(Error -36) Can't copy files off an external hard drive

Hi, I'm having trouble with my 250gb external drive.
I can copy files onto it fine, backed up all my music etc. but when I try to copy it back off I get an Error code -36 "Some of the data could not be read or written to".
I've tried formatting and re-formatting, writing zeros to the disk and tried formatting it to NTFS on a Windows PC but still no luck. Some very small files will copy off like text documents but that's about it.

I don't know if maybe upgrading to 10.5 will make any difference, just to make sure - I can definately upgrade to Leopard keeping all my files/settings?

Any help is much appreciated, thanks!

Try erasing the drive when connected through Disk Utility, go to partitions and select one partition and format Mac OS Extended (Journaled) and see if SuperDuper will copy to it, and you should then be able to copy back.